Jars and similar containers which are packed with a food product under vacuum (having less than atmospheric pressure in the head space above the food product) are more difficult to open than those which are not packed under vacuum. Atmospheric pressure above the closure exceeds the pressure in the head space beneath the closure, so that a net pressure differential force acts downwardly on the closure to hold it on the container. Since this pressure force is proportional to the area of the mouth of the container, it increases as the square of the radius of the container increases. Moreover, this pressure force rapidly increases as the size of the mouth of the container increases. Because the pressure force acts in conjunction with the frictional force of the closure threads, lugs, snaps, or other securing means to hold the closure on the container, it is much more difficult to remove a closure on a vacuum packed product. If the closure is a unitary, i.e., having an integral top panel screw thread closure, the mechanical friction between it and the container threads and the vacuum force must be overcome simultaneously. This occurs with these types of closures as the closures are rotated off the containers.
So-called composite closures, in which a separate insert disk or lid is rotatable within a threaded skirt or shell, facilitate opening vacuum packed containers because the shell can be rotated on the container to overcome the starting or mechanical friction without at the same time rotating the disk on the top or breaking the vacuum. Once the starting or mechanical friction is overcome, the threads gradually lift the disk and break the vacuum.
However, the problem is more difficult with closures of the so-called "press-on, pry-off" type, which are not removed by rotation. Such closures have a protrusion inside the shell which snaps beneath a snap rib on the container finish to secure the closure. Since no threads or lugs provide a mechanical advantage to lift the insert disk, the closure must be removed by prying it upwardly, as with a thumb positioned on its lower edge or an outwardly projecting tab, so as to force the shell protrusion over the snap rib. Both the pressure differential force and the tension of the snap must also be simultaneously overcome by the upward lifting force. Indeed, the required lifting force is so great that press-on, pry-off closures are impractical for some vacuum packed product containers, especially if the container's mouth diameters are greater than about 72 mm., unless an outwardly extending thumb tab is provided to give the needed leverage.
The force required to press open a press-on, pry-off closure is greater still if a tamper evidencing band is present. Such bands are designed to break or tear away before the closure can be opened or the contents interfered with, and are widely used to provide a visible indication if the closure has been partially or fully opened. Breaking the band adds another resistance which must be overcome, and thus further increases the pry-open force required.
In addition to the above-mentioned problems associated with removing closures from vacuum-packed containers, there is the ever increasing problem of a "dirty finish" on the container outer necks and snap ribs resulting from faster container filling and capping speeds. In other words, as the container filling and capping speeds increase, the more likely it is that the contents, such as food products, with which the containers are to be filled, will spill or splash onto the outer necks and snap ribs of the containers.
This "dirty finish" is also a common occurrence with those containers that are subject to retorting, i.e., following container filling and capping, food or vacuum-packed containers are cooked to temperatures on the order of about 250.degree. F. to sterilize the food contents filled therein. During the retorting process, if there is less than, for example, a 6% head space left in the containers due to overfill or if there is too much pressure within or too little pressure outside of the food or vacuum-packed containers, the hydraulic forces within the containers will cause vacuum seals to break and the inner food contents to seep between the container rims and the closures and then onto the containers' outer necks and snap ribs.
In either situation, the "dirty finish" on the container necks and snap ribs presents a sanitation problem if the spilled, splashed or seeped materials are permitted to remain and dry thereon. For example, if certain food residues are left on the outer necks and ribs of the containers to dry, mold growth, entrapped moisture, infestation of fruit flies or the like can result. Thus, a "clean finish" on the outer necks and snap ribs of the containers is required in order to meet and pass the USDA's FSIS requirements.
Still further, due to environmental concerns, there is an ever increasing demand to recycle plastic and glass containers such as described above. There is also an ever increasing demand to provide containers such as those described above with tamper evidencing indicators to advise consumers in advance as to whether the containers have or have not been tampered with. This is generally accomplished by providing the closures for containers with tamper evidencing bands which break free from the closures upon removal of the closures from the containers to open the containers. Unfortunately, a drawback associated with tamper indicating bands available heretofore is that, once they have been broken free from the closures, they remain secured around the necks concerning the containers. This drawback presents a recycling problem concerning the used containers, and in particular used glass containers, since the tamper evidencing bands which are secured to the container necks must be first cut free and removed therefrom before the used containers can be recycled.
Consequently, there is a demand in the industry for press-on, pry-off closures which can be more easily removed from containers by consumers. In addition, there is a demand in the industry for capped closures which have "clean finishes" on their outer necks and snap ridges following the filling and capping procedures as well as the retorting process. Still further, there is a demand in the industry for closures which facilitate the removal of the tamper indicating bands from used containers to simplify the use, reuse and recycling process of the used containers.